The present invention relates to a process for the thermal cracking of hydrocarbons contained in raw oils and an apparatus therefor. More particularly, the present invention relates to a process for the thermal cracking of hydrocarbons with a fused salt and steam to obtain lower olefins such as ethylene and propylene, and an apparatus therefor.
Olefin gases such as ethylene and propylene used as fundamental starting materials in the petroleum chemical industry have been produced by thermal cracking of light hydrocarbons such as naphtha using, for example, a tubular furnace of external heating type. It has been known that in the process wherein the tubular furnace of external heating type is used, coke is by-produced and thereby deposits in the device to cause troubles even though the light hydrocarbons are used. The troubles are serious when hydrocarbons such as kerosine, gas oil, heavy oil, asphalt oil and crude oil are thermally cracked by this process and it is difficult to put this process to practical use.
In the production of lower olefins by cracking ethane and naphtha, those raw hydrocarbons are mixed with steam and introduced into a reaction tube of cracking furnace. The fluid mixture is heated by plural burners arranged on the wall of the cracking furnace and thereby cracked to form a gas rich in lower olefins.
The cracking gas discharged from the reaction tube of cracking furnace is quenched in a quench heat exchanger in order to inhibit undesirable reaction such as polymerization reaction. One of defects caused in such an apparatus is that tars and coke by-produced by the thermal cracking of the raw hydrocarbons deposit in the reaction tube of the cracking furnace, heat transfer pipe of the quench heat exchanger and pipes connecting them to cause so-called coking phenomenon. Therefore, after continuation of the operation for a long period of time, they are accumulated on the tubular walls, whereby wall temperatures of the reaction tube of the cracking furnace and heat-transfer pipes of the quench heat exchanger are elevated to invite pressure loss and reduction in heat transfer efficiency. Consequently, the feeding of the raw hydrocarbons must be stopped to remove the deposits at regular intervals. In case hydrocarbon fractions such as crude oil, kerosine, gas oil, heavy oil and asphalt oil are cracked by this process, tar and coke deposits are by-produced in far larger amounts than those by-produced in case of using light hydrocarbon materials such as ethane and naphtha to make the long period operation of the apparatus utterly impossible.
Though the process for the thermal cracking of raw oils in the reaction tube of external heating type has the defects as above described, it has been attempted to thermally crack hydrocarbons using a fused salt or a fused metal as heat transfer medium (see, for example, the specifications of Japanese Patent Publications Nos. 29824/1964 and 5656/1963 and U.S. Pat. No. 3,252,774). However, according to those processes wherein heat of reaction required for the thermal cracking of hydrocarbons is furnished by sensible heat of the fused salt or fused metal, a large amount of the fused salt or fused metal of a high temperature must be kept in the system. This is unfavorable from the viewpoint of safety. Further, difficulties cannot be avoided in the design of the apparatus and also in the operations such as starting up and shutting down, since a large amount of the fused salt or fused metal is circulated in the reaction zone, regeneration zone and heating zone.
More particularly, processes for preventing coke deposition on the inner walls of the thermal cracking reaction tube or indirect heat exchanger by means of the molten metal are disclosed in Japanese Patent Publications Nos. 19244/1972, 8711/1975 and 41027/1974. However, in those processes, considerations are taken only of wetting property of the fused metal, structure of an inlet for the fused metal and materials of the inner walls. They are silent on effective amount of the fused metal, though amount of by-produced coke is varied depending on raw hydrocarbons used. Thus, they are unfavorable from viewpoints of economization, operation capacity and safety.